Tate Britain has been suffering an identity crisis for a decade, and is embarking on yet another round of therapy. The latest rehang of the collection is being billed as the beginning of an entirely new approach, scheduled to culminate in 2013 when a building programme gives it expanded gallery space and the entire collection will be displayed in chronological order. Yet if that sounds like a cure, the immediate arrangement pulls the patient to the very brink of catastrophe.

Like those of some hapless royal in search of a "role", Tate Britain's troubles over what exactly it's for boil down to the quarrel of ancients and moderns. Does this British part of the Tate empire exist to celebrate and display the history of British art since the Renaissance, or is it just a showcase for contemporary British art? The new displays lurch towards the latter, but in a dull way that drags the original young artists of the 90s into an establishment history of British modernism – when in fact they deserve to be seen with British greats like Hogarth and Turner.

Surely I can't be the only visitor who, entering a museum billed online as "the home of British art from 1500 to the present day", expects to see just that: a rich array of British art from Rubens' oil sketch for the Banqueting House ceiling – saved for the Tate Collection after a public appeal in 2008 – to the latest young artists.

Yet room after room of the new display reveals more and more British 20th-century art – the good, the bad and the indifferent. Last time I checked we were in the 21st century, and it makes no more sense to insist on the relevance of the period 1900-2000 than to dedicate the entire museum to, say, the 16th century. Though it seems there's fat chance of the latter. The Tate press office insists that a third of the art on display is still pre-1900, but most of such older works as are visible will be crammed into one big room of "iconic" paintings or shoved into the Clore gallery, which is supposed to be dedicated to Turner. Using the press office's own figures, that means four-fifths of the collection (ie, 1500-1900) gets just a third of the space.

Even if the new displays are the aperitif to a great new museum in 2013, it is simply wrong to put so many works of deep historical and often aesthetic interest into storage for three years, or two years, or one year. There are plenty of iconoclasts in the wings asking why public collections don't sell off a few old paintings, and frankly this lack of regard for national treasures gives them credence.

Are we all supposed to simply forget British history for the next couple of years? If I were a teacher, I'd take my class to the V&A instead, where the past, present and future all flourish, as they should in any museum worthy of the name.